The electron linear accelerator is a device which accelerates electrons under a microwave electromagnetic field so that the energy of the electrons is enhanced. An electron beam generated by using the accelerator has a widespread application prospect, such as medical treatment, irradiation, imaging, etc.
In order to acquire maximum acceleration efficiency, all conventional electron linear accelerators are designed so that a variation in a phase velocity of the microwave is consistent with a variation in a movement velocity of the accelerated electrons. According to the theory of relativity, as the energy of the electrons has improved, the movement velocity of the electrons approaches the light velocity rapidly. Therefore, when a conventional low-energy electron linear accelerator is designed, it is generally divided into a bunching section and a light velocity section. In the bunching section, the phase velocity of the microwave increases slowly. A variation in the phase velocity is substantially the same as a variation in the velocity of the electrons, to ensure particular capture efficiency and an energy spectrum. In the light velocity section, the phase velocity is equal to the light velocity, and the movement velocity of the electrons also approaches the light velocity. Therefore, the electrons are also synchronous with the microwave, and the phase of the electrons is near the maximum accelerating phase, to obtain optimal accelerating efficiency.
In general, the output energy of the electron linear accelerator is fixed. However, in practical applications, it is generally desired to adjust the energy of the accelerator as needed. In order to adapt to the requirements of the practical applications, various methods for adjusting energy appear successively. At present, methods for adjusting energy commonly used by the electron linear accelerator comprise:
(1) changing an overall electromagnetic field distribution of the accelerating tube. This is generally implemented by regulating feed-in microwave power or a beam load. This method is relatively easy to implement, but in order to ensure that the capture efficiency and beam energy spectrum of the bunching section, the variation of the field amplitude can not be too large, and therefore the energy regulating range is limited.
(2) keeping the field amplitude of the bunching section substantially unchanged, and individually changing a field amplitude or phase of the light velocity section. There are generally two implementation manners for this solution at present: one is to feed energy into the bunching section and the light velocity section separately to achieve the purpose of independent regulation, such as U.S. Pat. No. 2,920,288, U.S. Pat. No. 3,070,726 and U.S. Pat. No. 4,118,653; and the other is to regulate a field amplitude ratio or phase relationship between the bunching section and the light velocity section by an energy switch, such as U.S. Pat. No. 4,286,192 and CN patent CN 1102829 C. This method can obtain a relatively large energy regulating range, but the structure of the microwave feed-in system or the accelerator is relatively complex.